Many microwave circuits depend on a reference oscillator constructed from an amplifier and a high Q resonator. There has been a long felt need for simple resonators with extremely high Q values. Simple metal cavities were the original prototype microwave resonators. The performance of these devices, however, is limited by the resistive losses in the enclosure walls. These losses result from currents generated in the walls by the electromagnetic fields in the cavity.
To reduce such losses, a low loss dielectric material is placed in the cavity. The material partially confines the electromagnetic fields within the dielectric, and thus decreases the relative strength of the fields and shielding currents at the enclosure walls. This resonator configuration is often referred to as a "loaded cavity" configuration. The performance limitations of loaded cavity resonators are governed by a combination of the loss tangent of the dielectric material, and the (reduced) currents in the enclosure walls.
The dielectric with the lowest known loss tangent at microwave frequencies is monocrystalline sapphire. Furthermore, the loss tangent of this material decreases with temperature, being proportional to T.sup.-5. As a result, it is the material of choice for many high performance applications of dielectric resonators, particularly at cryogenic temperatures.
Unfortunately, sapphire dielectric resonators have a relatively low dielectric constant (K=10). As a result, the strength of the electromagnetic field external to the dielectric itself is still relatively high compared to materials with significantly higher dielectric constants. Thus, specialized design strategies are needed to reduce the losses due to the contributions from the electrical currents on the inside surfaces of the enclosure walls. In order to address this issue, two generic varieties of sapphire dielectric resonators have been extensively studied. The first type is a low order TE mode resonance of a cylindrical sapphire "puck" centered in a cylindrical conducting enclosure, and the second is the so-called "whispering gallery" resonator.
This first generic style of resonator generally has a performance limited by the enclosure wall loss, due to relatively weak confinement of the electromagnetic fields to the dielectric puck. Some improvement in Q values can be achieved by replacing the conducting end caps of the enclosure by high temperature super-conducting films. This reduces resistive losses, although in all reported work on sapphire dielectric resonators of this configuration, wall losses remain the dominant source of Q limitations. Another major drawback of this style of resonator is the extreme vibration sensitivity exhibited by such devices. This sensitivity results from the significant residual field at the enclosure walls for the mode in which the cavity operates. This leads to substantial mode variation when there is relative motion between the dielectric puck and the enclosure. This microphonic sensitivity causes severe degradation of the phase noise performance at low offset frequencies for oscillators based on dielectric resonators of this type.
The second generic type of sapphire dielectric resonator is the whispering gallery mode configuration. This type of resonator consists of a sapphire ring or disk which confines the electromagnetic energy to the dielectric region by a physical mechanism not unlike total internal reflection in optical systems. The modes can be qualitatively described as traveling waves around a bent dielectric waveguide which closes upon itself. This configuration provides strong electromagnetic energy confinement due to the existence of only evanescent fields outside of the dielectric structure. The relative field confinement increases rapidly as a function of mode number. Here, mode number is defined as the number of modal maxima encountered in one complete circuit of the ring. For mode numbers on the order of 7 or more, the relative field energy confinement is sufficient to significantly reduce the Q degradation from enclosure wall loss relative to the first type of resonator described above. Configurations of this type allow Q values approaching the limitations imposed by the internal sapphire loss mechanisms, as well as greatly reduced vibrational sensitivity.
Unfortunately, at microwave frequencies, the mode density in the frequency domain is extremely high. Hence, the desired resonant mode is typically very close in frequency to other, spurious, modes. These spurious modes consist of other whispering gallery modes as well as "hybrid" modes, which are simply the usual empty cavity modes perturbed by the presence of the sapphire ring. In prior art systems, the high mode density of the whispering gallery resonators has precluded their use in a simple flee-running oscillator with direct RF feedback because of the inherent difficulty of forcing the circuit to oscillate at only the desired resonance frequency.
As a result, prior art systems based on whispering gallery resonators have typically been limited to uses in which the whispering gallery resonator is used as a frequency discriminator to stabilize a lower performance local oscillator. This application is often feasible because the intrinsic stability of the local oscillator is usually sufficient to select the desired whispering gallery mode from the "dense forest" of modes in the resonator spectrum. Some work has also been done in attempting to clean up the dense mode spectrum by iteratively measuring, and then modifying, the resonator enclosure using microwave absorbers and frequency tuners to reduce the Q and coupling to the undesired modes. While this approach has reduced the number of undesired modes, it requires a tedious design procedure and still does not allow incorporation of a whispering gallery mode resonator into a simple direct feedback free-running oscillator. Consequently, the full benefits promised by the high Q and low vibration sensitivity of the whispering gallery resonator had not been realized in prior art systems.
Another problem that has prevented prior art whispering gallery resonators from being used in direct-feedback free-running oscillators has been the temperature coefficient of the dielectric constant. If the dielectric constant were independent of temperature, it would be possible to construct oscillators with center frequencies that were nearly independent of temperature. One prior art approach to solving this problem is to use a composite material that has an essentially zero temperature coefficient at the desired operating temperature. For example, some of the materials that have been commonly used for dielectric resonators are ceramics that have been produced from mixtures of components that have been carefully chosen to produce a zero temperature coefficient of the dielectric constant at some selected temperature.
Unfortunately, this approach cannot be used with sapphire, the substance of choice at microwave frequencies, since there is no way to directly influence the temperature coefficient of its dielectric constant without significantly degrading its loss tangent. At room temperature, the dielectric constant of sapphire changes by roughly 70 parts per million per degree K. While this value decreases approximately in proportion to T.sup.3, the temperature dependence still presents a serious obstacle to stable oscillator operation even at temperatures as low as 20.degree. K.
Yet another problem with whispering gallery resonators is a result of the fact that even for an ideal system, the modes of the whispering gallery resonator actually occur in perfectly degenerate pairs. The whispering gallery modes can be qualitatively described as traveling waves around a bent dielectric waveguide which closes upon itself, with the energy confinement and guiding occurring by a physical mechanism not unlike total internal reflection in optical systems. The symmetry of the system allows traveling waves to propagate in either direction around the ring, leading to a two-fold degeneracy of all whispering gallery modes. In actual practice, the two degenerate modes are not counter-propagating traveling waves, but rather standing waves rotated spatially by one quarter wavelength from one another.
Because of imperfections in real devices, as well as the non-zero size of the physical coupling loops used to transfer energy to and from the cavity, the nominally degenerate modes are perturbatively coupled, and their frequencies slightly separated. In addition, the coupling to the mode which has a magnetic field maximum at the azimuthal position of the (inductive) coupling loop is much stronger than that of the mode rotated by a quarter wavelength.
Unfortunately, this circumstantial frequency splitting, and preferential selective mode coupling are not enough to produce adequate mode separation. One crude method for forcing a frequency separation between the modes is to extend a very thin bar of sapphire through the enclosure wall, allowing it to press against the sapphire ring. If the point of contact is at a field null of the desired whispering gallery mode, and at a field maximum of its undesired degenerate sister-mode, the undesired mode will be strongly perturbed to a lower frequency due to dielectric loading. In contrast, the desired mode is relatively unchanged. Unfortunately, this technique has a number of drawbacks. First, the technique requires a fairly sensitive manual tuning of each device which increases the cost of the devices. Second, the offset of the undesired mode in frequency is relatively small and not well controlled. Finally, an additional piece of machined sapphire is required and the cavity must be provided with an additional opening and mechanisms for positioning the piece of sapphire relative to the resonator ring.
Yet another problem inherent in constructing high Q whispering gallery resonators is the method of supporting the resonator in order to maintain the desired geometry. If the resonator is placed firmly against one wall of the enclosure, most of the benefits of high mode confinement are lost. Thus, it is necessary to devise a mechanical support for the dielectric ring which appropriately addresses the mechanical, electromagnetic and thermal requirements of the resonator system.
These requirements are typically in conflict with each other. For the best mechanical properties, the support arrangement should be short and relatively massive, while the electromagnetic properties are optimized if the support is constructed from long, thin components with minimal cross-section. There are two key considerations relevant to the electromagnetic properties of the support. First, it is essential that the support not introduce extra loss. Typically, this will restrict the choice of materials for the support. A dielectric with low dielectric constant and minimal loss tangent is needed. Second, the support must avoid disrupting any of the electromagnetic modes of the resonator and its enclosure.
The support problem is particularly severe in the case of low dielectric constant resonators such as resonators made from sapphire. It should be noted that these problems are much less severe for dielectric resonators constructed from materials with a comparatively high dielectric constant. In such cases, a support material with a relatively much lower dielectric constant exists and can be used. Such a material will have a correspondingly low loss tangent which will not significantly influence the electromagnetic mode shape or dissipation. However, for dielectric resonator design, the materials with the lowest known loss tangents are sapphire and quartz, and these both have low dielectric constants.
Broadly, it is the object of the present invention to provide an improved whispering gallery resonator.
It is a further object of the present invention to provide a whispering gallery resonator in which the mode spacing around the mode of choice is sufficient to allow the resonator to be used in a simple direct feedback free-running oscillator.
It is a still further object of the present invention to provide a whispering gallery resonator which is less sensitive to external temperature fluctuations than prior art whispering gallery resonators.
It is yet another object of a pair of present invention to provide a whispering gallery resonator in which one of the degenerate modes is substantially shifted in frequency relative to the other.
It is a still further object of the present invention to provide a whispering gallery resonator with a support system having a low loss tangent while providing adequate rigidity.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.